During the past several years, the popularity and viability of fuel cells for producing both large and small amounts of electricity has increased significantly. Fuel cells conduct an electrochemical reaction with reactants such as hydrogen and oxygen to produce electricity and heat. Fuel cells are similar to batteries except fuel cells can be “recharged” while providing power. In addition, fuel cells are cleaner than other sources of power, such as devices that combust hydrocarbons.
Fuel cells provide a DC (direct current) voltage that may be used to power motors, lights, computers, or any number of electrical appliances. A typical fuel cell includes an electrolyte disposed between an anode and a cathode. An electrochemical reaction is conducted in the fuel cell in which an oxidant, such as air, is fed to the cathode. From the incoming air, the cathode supplies oxygen ions to the electrolyte. A fuel such as hydrogen or methane is fed to the anode where it is transported to the electrolyte to react with the oxygen ions. This reaction produces electrons, which are then introduced into an external circuit as useful power.
Not all fuel introduced into a fuel cell system will be utilized. This incomplete utilization is due, at least in part, to depletion effects. Depletion effects are the decreasing concentration of fuel in a fuel stream as it passes over a fuel cell stack in the flow direction. Depletion effects result in a higher amount of heat at the fuel entrance that decreases as the fuel passes over the fuel cell stack in the flow direction. This heat generation profile causes a corresponding temperature gradient in the fuel cell stack. This temperature gradient may contribute to a decreasing power production profile of the individual fuel cells corresponding to the temperature gradient due to the correlation between operating temperature and power production.
In addition, typical systems currently do little to manage cathode airflow over the fuel cell stack. Typically the airflow is approximated by a plug flow. Plug flow is a substantially uniform amount of airflow entering one end of flow duct having a uniform cross section and exiting at the other end with no augmentation along the flow path. Plug flow results in constant velocity over each cross section of flow in the duct. As a result, the convective heat transfer from the fuel cell stack to the cathode air is hindered by the rising free stream temperature as the air flows along the length of the fuel cell stack. This phenomenon adds to thermal gradients occurring across the cell. In order to minimize this effect and to provide the required cooling for the cell, current systems use up to 700 to 800% of the air that is actually required stoichiometrically for the reaction. Moving this volume of air requires larger pumps, more power to run those pumps, and larger valves. This, in turn, leads to higher operating costs.